1. Field of the Invention
This invention relates to a circuit for sensing the change in capacitance of variable capacitor and, more particularly, to a sensor circuit for detecting excessive amounts of water in the fuel tank of a vehicle by sensing the capacitance change of a probe capacitor positioned within the fuel tank.
2. Description of the Prior Art
In attempt to conserve the earth's limited reserve of fossil fuel, many automotive manufacturers are increasingly building more and more automobiles that are driven by diesel fuel supplied internal combustion engines. Diesel engines are generally more fuel efficient than relatively sized gasoline operated engines. A problem related more to diesel engines arises if water is allowed to accumulate in the diesel fuel. Due to the nature of the diesel engine, water in the fuel can damage or even destroy the fuel pump, fuel system and or fuel injectors of the diesel engine.
Until recently diesel engines were mainly used only in commercial vehicles such as earth moving equipment, buses, etc. and very expensive automobiles. Because of the expensive nature of these vehicles, complex and expensive filter systems could be utilized to inhibit water from getting into the diesel fuel. However, as diesel engines are now being used in massed produced consumer oriented automobiles, the automotive manufacturers are in need of a simple and reliable sensor circuit that can detect excessive amounts of water in the fuel such that the operator can be alerted to this condition.
Generally, it is desired to have a dash mounted panel light that can be lit up on the dash panel of the vehicle to warn the operator of water in the fuel tank. The operator would as soon as possible thereafter have the fuel tank drained to remove the excess water.
It is generally known that water can be differentiated from the diesel fuel by its dielectric constant. Since water does not homogeneously mix with the fuel but rather goes to the bottom of the fuel tank, it's presence can be determined by utilizing a capacitor which varies in value as water is displaced thereabout and comparing it's value to a reference capacitor. In fact, at least one automobile manufacturer uses such a scheme for detecting water in the fuel. Basically, this prior art scheme uses an oscillator to regularly charge up both capacitors while allowing the capacitors to discharge through fixed value resistors while at the same time comparing the voltages developed there across. Water causes an increase in the capacitor size of the variable capacitor and thus the voltage developed thereacross which eventually will cause the output of the comparator to go positive at a predetermined point. This output is then rectified and filtered to be used to turn on a warning lamp.
Several problems are associated with this prior art system. The system is relatively expensive, utilizing building block integrated circuits in combination with discrete devices. Additionally, the system suffers in its accuracy. Hence, a need exists for such a system which is suitable to be manufactured in integrated circuit form to reduce system cost while increasing system accuracy.